This invention relates in one aspect to a method for generating pollution credits while processing molten reactive metals such as magnesium, aluminum, lithium, and alloys of such metals.
Molded parts made of magnesium (or its alloys) are finding increasing use as components in the automotive and aerospace industries. These parts are typically manufactured in a foundry, where the magnesium is heated to a molten state to a temperature as high as 1400xc2x0 F. (800xc2x0 C.), and the resulting molten magnesium is poured into molds or dies to form ingots or castings. During this casting process, protection of the magnesium from atmospheric air is essential to prevent a spontaneous exothermic reaction from occurring between the reactive metal and the oxygen in the air. Protection from air is also necessary to minimize the propensity of reactive magnesium vapors to sublime from the molten metal bath to cooler portions of a casting apparatus. In either situation, an extremely hot magnesium fire can result within a few seconds of air exposure, potentially causing extensive property damage and serious injury or loss of human life. Similarly, aluminum, lithium, and alloys of such metals are highly reactive in molten form necessitating protection from atmospheric air.
Various methods have been investigated to minimize the exposure of molten magnesium to air. See J. W. Fruehling et al., Transactions of the American Foundry Society, Proceeding of the 73rd Annual Meeting, May 5-9, 1969, 77 (1969). The two most viable methods for effectively separating molten magnesium from air are the use of salt fluxes and the use of cover gases (sometimes referred to as xe2x80x9cprotective atmospheresxe2x80x9d). A salt flux is fluid at the magnesium melt temperature and it effectively forms a thin impervious film on the surface of the magnesium, thus preventing the magnesium from reacting with oxygen in the air. However, the use of salt fluxes presents several disadvantages. First, the flux film itself can oxidize in the atmosphere to harden into a thick deposit of complex metal oxide/chlorides, which is easily cracked to expose molten magnesium to the atmosphere. Second, the salt fluxes are typically hygroscopic and, as such, can form salt inclusions in the metal surface which can lead to corrosion. Third, fumes and dust particles from fluxes can cause serious corrosion problems to ferrous metals in the foundry. Fourth, salt sludge can form in the bottom of the crucible. Fifth, and not least, removal of such fluxes from the surface of cast magnesium parts can be difficult.
As a result, there has been a shift from using salt fluxes to using cover gases to inert molten magnesium. Cover gases can be described as one of two types: inert cover gases and reactive cover gases. Inert cover gases can be non-reactive (e.g., argon or helium) or slowly reactive (e.g., nitrogen, which reacts slowly with molten magnesium to form Mg3N2). For inert cover gases to be effective, air must be essentially excluded to minimize the possibility of metal ignition, i.e., the system must be essentially closed. To utilize such a closed system, workers either have to be equipped with a cumbersome self-contained breathing apparatus or they have to be located outside of the dimensions of the processing area (e.g., by using remote control). Another limitation of inert cover gases is that they are incapable of preventing molten metal from subliming.
Reactive cover gases are gases used at low concentration in a carrier gas, normally ambient air, that react with the molten magnesium at its surface to produce a nearly invisible, thermodynamically stable film. By forming such a tight film, the aerial oxygen is effectively separated from the surface of the molten magnesium, thus preventing metal ignition and minimizing metal sublimation.
The use of various reactive cover gases to protect molten magnesium from ignition has been investigated as early as the late 1920s. An atmosphere containing CO2 is innocuous and economical yet forms a protective film on a magnesium surface which can prevent ignition for over 1 hour at 650 xc2x0 C. However, the CO2-based films formed are dull in appearance and unstable, especially in the presence of high levels of air, and consequently offer little protection for the magnesium surface from ambient oxygen. In effect, the CO2 behaves more like an inert cover gas than a reactive cover gas.
U.S. Pat. No. 4,770,697 (Zurecki) discloses the use of dichlorodifluoromethane as a blanketing atmosphere or cover gas for molten aluminum-lithium alloys. U.S. Pat. Nos. 6,398,844 and 6,521,018 (both Hobbs et al.) disclose blanketing gases used with non-ferrous metals and alloys with reduced Global Warming Potentials, but which are very toxic to workers and/or corrosive to process equipment.
SO2 has been investigated in the past as a reactive cover gas, as SO2 reacts with molten magnesium to form a thin, nearly invisible film of magnesium oxysulfides. SO2 is low in cost and is effective at levels of less than 1% in air in protecting molten magnesium from ignition. However, SO2 is very toxic and consequently requires significant measures to protect workers from exposure (permissible exposure levels are only 2 ppm by volume or 5 mg/m3 by volume). Another problem with SO2 is its reactivity with water in humid air to produce very corrosive acids (H2SO4 and H2SO3). These acids can attack unprotected workers and casting equipment, and they also contribute significantly to acid rain pollution when vented out of the foundry. SO2 also has a tendency to form reactive deposits with magnesium which produce metal eruptions from the furnace (especially when SO2 concentrations in the air are allowed to drift too high). Though SO2 has been used commercially on a large scale for the casting of magnesium alloys, these drawbacks have led some manufacturers to ban its use.
Fluorine-containing reactive cover gases provide an inert atmosphere which is normally very stable to chemical and thermal breakdown. However, such normally stable gases will decompose upon contact with a molten magnesium surface to form a thin, thermodynamically stable magnesium oxide/fluoride protective film. U.S. Pat. No. 1,972,317 (Reimers et. al.) describes the use of fluorine-containing compounds which boil, sublime or decompose at temperatures below about 750xc2x0 C. to produce a fluorine-containing atmosphere which inhibits the oxidation of molten magnesium. Suitable compounds listed include gases, liquids or solids such as BF3, NF3, SiF4, PF5, SF6, SO2F2, (CClF2)2, HF, NH4F and NH4PF6. The use of BF3, SF6, CF4 and (CClF2)2 as fluorine-containing reactive cover gases is disclosed in J. W. Fruehling et al., described supra.
Each of these fluorine-containing compounds has one or more deficiencies. Though used commercially and effectively at lower levels than SO2, BF3 is toxic and corrosive and can be potentially explosive with molten magnesium. NF3, SiF4, PF5, SO2F2 and HF are also toxic and corrosive. NH4F and NH4PF6 are solids which sublime upon heating to form toxic and corrosive vapors. CF4 has a very long atmospheric lifetime. (CClF2)2, a chlorofluorocarbon, has a very high ozone depletion potential (ODP). The ODP of a compound is usually defined as the total steady-state ozone destruction, vertically integrated over the stratosphere, resulting from the unit mass emission of that compound relative to that for a unit mass emission of CFC-11 (CCl3F). See Seinfeld, J. H. and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley and Sons, Inc., New York, (1998). Currently, there are efforts underway to phase out the production of substances that have high ODPs, including chlorofluorocarbons and HCFCs, in accordance with the Montreal Protocol. UNEP (United Nations Environment Programme), Montreal Protocol on Substances that Deplete the Ozone Layer and its attendant amendments, Nairobi, Kenya, (1987).
Until recently, SF6 was considered the optimum reactive cover gas for magnesium. SF6 is effective yet safe (essentially inert, odorless, low in toxicity, nonflammable and not corrosive to equipment). It can be used effectively at low concentrations either in air ( less than 1%) or in CO2 to form a very thin film of magnesium oxyfluorides and oxysulfides on the surface of molten magnesium. This magnesium oxide/fluoride/sulfide/sulfur oxide film is far superior at protecting the magnesium from a vigorous exothermic oxidation reaction than is the magnesium oxide film inherently present on the metal surface. The magnesium oxide/fluoride/sulfide/sulfur oxide film is sufficiently thin (i.e., nearly invisible to the naked eye) that the metal surface appears to be metallic. This superior protection is believed to result from the greater thermodynamic stability of a nonporous magnesium sulfide/sulfur oxide and/or magnesium oxide/fluoride film as compared to the stability of a thick porous film of either magnesium oxide, sulfide or fluoride alone.
In a typical molten magnesium process employing a reactive cover gas, only a small portion of the gas passed over the molten magnesium is actually consumed to form that film, with the remaining gas being exhausted to the atmosphere. Efforts to capture and recycle the excess SF6 are difficult and expensive due to its very low concentrations in the high volumes of exhaust stream. Efficient thermal oxidizing equipment would be required to remove the SF6 from the exhaust stream, adding significantly to production costs. Product costs can also be considerable, as SF6 is the most expensive commercially used reactive cover gas.
However, perhaps the greatest concern with SF6 is its very significant global warming potential (3200 year atmospheric lifetime, and about 22,200 times the global warming potential of carbon dioxide). At the December 1997 Kyoto Summit in Japan, representatives from 160 countries drafted a legally binding agreement containing limits for greenhouse gas emissions. The agreement covers six gases, including SF6, and includes a commitment to lower the total emissions of these gases by the year 2010 to levels 5.2% below their total emissions in 1990. UNEP (United Nations Environment Programme), Kyoto Protocol to the United Nations Framework Convention on Climate Change, Nairobi, Kenya, 1997.
As no new replacement for SF6 is yet commercially available, efforts are underway to reinvigorate SO2, as SO2 has essentially no global warming potential (despite its other considerable drawbacks). See H. Gjestland, P. Bakke, H. Westengen, and D. Magers, Gas protection of molten magnesium alloys: SO2 as a replacement for SF6. Presented at conference on Metallurgie du Magnesium et Recherche d""Allegement dans I""""Industrie des Transports, International Magnesium Association (IMA) and Pole de Recherche et de Devleoppment Industriel du Magnesium (PREDIMAG) Clermond-Ferrand, France, October 1996.
The data in TABLE 1 summarize selected safety and environmental limitations of compounds currently known to be useful in the protection of molten magnesium. Numbers followed by an asterisk (*) are particularly problematic with regard to safety and/or environmental effects.
As each of these compounds presents either a significant safety or an environmental concern, the search continues to identify new reactive cover gases for protecting molten magnesium, aluminum, lithium, and alloys of such metals which are simultaneously effective, safe, environmentally acceptable, and cost-effective.
This invention relates in one aspect to a method for generating pollution credits while processing molten reactive metals and alloys of such metals, e.g., magnesium, aluminum, lithium, and alloys of one or more of such metals. Reactive metals are metals (and alloys) which are sensitive to destructive, vigorous oxidation in air. In brief summary, the invention provides a method for generating pollution credits comprising:
(a) treating molten reactive metal or alloy of such metal to protect said metal or alloy from reacting with oxygen in air by (1) providing molten metal or alloy and (2) exposing said metal or alloy to a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof to yield protected metal or alloy having a protective film thereon; and
(b) taking allocation of pollution credits.
In one embodiment, this invention employs a method for treating molten reactive metal or alloy to protect it from reacting with oxygen in air. The method comprises providing molten reactive metal or alloy and exposing it to a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof. The gaseous mixture may further comprise a carrier gas. The carrier gas may be selected from the group consisting of air, carbon dioxide, argon, nitrogen and mixtures thereof.
One advantage of the present invention over the known art is that the Global Warming Potentials of perfluoroketones and hydrofluoroketones are quite low. Therefore, the present inventive process is more environmentally friendly. By employing the method for treating or protecting molten reactive metals or alloys which is described herein, processors who handle molten reactive metals or alloys will be able to produce unit quantities of such metals and alloys and parts containing such metals and alloys as before while generating much smaller quantities of materials exhibiting significant GWP contribution or other environmentally desirable effect.